Radiative forcing continues to be a useful concept, providing a convenient
first-order measure of the relative climatic importance of different agents
(SAR; Shine and Forster, 1999). It is computationally much more efficient than
a GCM calculation of the climate response to a specific forcing; the simplicity
of the calculation allows for sophisticated, highly accurate radiation schemes,
yielding accurate forcing estimates; the simplicity also allows for a relative
ease in conducting model intercomparisons; it yields a first-order perspective
that can then be used as a basis for more elaborate GCM investigations; it potentially
bypasses the complex tasks of running and analysing equilibrium-response GCM
integrations; it is useful for isolating errors and uncertainties due to radiative
aspects of the problem.

In gauging the relative climatic significance of different forcings, an important
question is whether they have similar climate sensitivities. As discussed in
Section 6.2.1, while models indicate a reasonable
similarity of climate sensitivities for spatially homogeneous forcings (e.g.,
CO2 changes, solar irradiance changes), it is not possible as yet
to make a generalisation applicable to all the spatially inhomogeneous forcing
types. In some cases, the climate sensitivity differs significantly from that
for CO2 changes while, for some other cases, detailed studies have
yet to be conducted. A related question is whether the linear additivity concept
mentioned above can be extended to include all of the relevant forcings, such
that the sum of the responses to the individual forcings yields the correct
total climate response. As stated above, such tests have been conducted only
for limited subsets of the relevant forcings.

Another important limitation of the concept is that there are parameters other
than global mean surface temperature that need to be determined, and that are
as important from a climate and societal impacts perspective; the forcing concept
cannot provide estimates for such climate parameters as directly as for the
global mean surface temperature response. There has been considerably less research
on the relationship of the equilibrium response in such parameters as precipitation,
ice extent, sea level, etc., to the imposed radiative forcing.

Although the radiative forcing concept was originally formulated for the global,
annual mean climate system, over the past decade, it has been extended to smaller
spatial domains (zonal mean), and smaller time-averaging periods (seasons) in
order to deal with short-lived species that have a distinct geographical and
seasonal character, e.g., aerosols and O3 (see also the SAR). The
global, annual average forcing estimate for these species masks the inhomogeneity
in the problem such that the anticipated global mean response (via Equation
6.1) may not be adequate for gauging the spatial pattern of the actual climate
change. For these classes of radiative perturbations, it is incorrect to assume
that the characteristics of the responses would be necessarily co-located with
the forcing, or that the magnitudes would follow the forcing patterns exactly
(e.g., Cox et al., 1995; Ramaswamy and Chen, 1997b).